1. Field of the Invention
This invention relates to a method of treating clay-containing phosphate rock. More particularly, this invention relates to a method of treating clay-containing phosphate rock to control problems associated with swelling of the clays when admixed with water. In particular embodiments, this invention relates to treatment of admixture of water and clay-containing phosphate rock, with or without added acid, to provide a plurality of benefits, including controlling problems associated with intolerable build up of viscosity because of swelling of the clays.
2. Description of the Prior Art
The prior art has seen a wide variety of processes involving phosphate rock. These processes have ranged from merely distributing the phosphate rock powder as a fertilizer in the early days of antiquity through the present sophisticated processes for beneficiating the phosphate rock to upgrade its phosphate content and processing through a wide variety of steps, including the addition of water either before or after comminution, or grinding the phosphate rock to a predetermined size range. The processes may be employed in making fertilizers of various sorts up to and including the addition of acid, such as sulfuric acid, for the manufacture of phosphoric acid. During the past 20 years, various phosphate industries have made rapid strides in cutting the cost of production and have allowed new and important phosphorus-containing materials to be introduced. The phosphorus industry has proved itself to be one branch of inorganic chemistry that has not been overshadowed by the fast moving organic chemical developments of recent years and has, in fact, joined with the rapidly moving organic chemical field to form phosphorus-organic compounds that are of increasing industrial importance each year. The processes have become increasingly sophisticated, each degree of sophistication having its own problem. In the early days of the Davy Powergas-Prayon Modern Dihydrate Process it was reported that the wet grinding of a phosphate rock before addition of a concentrated acid had unique advantages that allowed employing slurries with high concentrations of solids therein and solved most of the problems of the prior art. In practice, however, the clays in the phosphate rock tended to increase intolerably the viscosities of the slurries, and papers have been given reporting these intolerable increases in viscosities. Such problems have negated the advantages of the Modern Dihydrate Process, at least in part, by requiring the rather continuous dilution back of the slurries so as to render them pumpable. This has created problems handling large volumes of water, required room for dilution and decreased the capacity of storage vats, required the separation of large volumes of water, and diluted the phosphoric acid formed, as well as diluted the reactant acid. Moreover, as has been previously reported in the patent literature, such as U.S. Pat. No. 3,192,014, there is a long-standing problem of producing calcium sulfate crystals that can be readily filtered from the liquor containing the phosphoric acid. The reason for the dilution back requirement is that, in order for a plant to operate efficiently, the ground phosphate rock in water slurry has to be stored for a prolonged interval in order to ensure efficient operation and not have peaks and valleys in the production schedule. It has been found that a storage capacity of about 4 hours will provide the requisite efficiency. Of course, longer storage can be employed, if desired, but this adds to the cost and is not necessary.
In summary, it has become apparent that some chemical treatment is needed to alleviate the problem with the intolerable increasing in viscosity of the stored slurry, in addition to alleviating the other problems of the prior art processes. Conventional dispersants cannot be employed satisfactorily to prevent this intolerable build up over such a protracted storage interval, and the prior art has not provided a solution to this problem.